This invention relates to a casing scraper, for cleaning the inner wall surfaces of a tubular member such as a bore casing or lining in an oil or gas well. The invention is not however restricted to this particular application.
During the drilling of an oil well, a casing is set into the ground and various drilling and cementing processes take place before the well is ready for production. Prior to production the well casing has to be cleaned to remove debris which may be stuck to the casing walls, resulting from some of the previous well preparation operations.
It is known to pass a casing scraper along the well. Such a scraper has spring-biased brushes or scraping tools which clean the inner surface of the casing as the scraper is moved up and down and rotated in the casing. Examples of such casing scrapers are shown, for example, in U.S. Pat. No. 4,479,538 and in U.S. Pat. No. 5,570,742.
According to the present invention, there is provided a casing scraper for cleaning the inner surface of a tube of a predetermined internal diameter, the scraper having an axis of rotation, a plurality of axially spaced, rigidly connected scraping surfaces with each surface having an angular extent of less than 180xc2x0 and being angularly offset from other surfaces, and shank portions rigidly connecting the scraping surfaces to one another, wherein prior to the insertion of the scraper in the tube, first distances, from the axis of rotation to each scraping surface, are greater than the radius of the internal diameter of the tube to be scraped, second distances from the axis of rotation to a surface diametrically opposite to each scraping surface, are less than the tube radius and the sum of the first distance and the second distance at each scraping surface is less than the internal tube diameter such that the shank portions are required to bend to enable the scraper to be admitted to the tube internal diameter.
The eccentric arrangement of the scraping surfaces, and the axial spacing between the surfaces causes the parts of the scraper connecting the scraping surfaces to be placed in bending when the scraper is in place within the tube. The bending or flexing of the intermediate parts between the scraping surfaces produces a stress (stored energy) which urges the scraping surfaces into contact with the tube internal surface. Because the centricity of the scraper is ensured by contact with the tube wall at at least three angularly spaced positions, all the scraping surfaces are positively urged against the tube internal surface, without the need for any relatively moving parts.
The scraping surfaces can be axially spaced by connecting shanks which are integral with the scraping surfaces, or by (modified) drillpipe connecting rods which can be screwed together before the scraper is used.
The scraping surfaces may have surface grooves in the form of a partial helical screw thread which engages with the wall of the tube to perform a scraping action. However other surface formations, or scraping tools such as brushes mounted on the scraper can form the scraping surfaces.
The scraping surfaces, considered together, preferably have an angular extent of 360xc2x0. This ensures that all parts of the tube wall are swept, even if the scraper is only moved axially, and not in rotation, as it moves along the tube.
In order to achieve the desired force or side wall loading of the scraping surfaces against the tube wall, the first distance can be 1.005 to 1.010 times the second distance. This relatively small difference between the scraper and the tube internal diameter is sufficient to exert the necessary force to achieve good scraping while allowing the scraper to enter the tube without undue difficulty and without incurring high friction loads between the scraper and the tube walls which could slow down scraping and increase the energy requirement to move the scraper through the tube.
The angular extent of each scraping surface can be between 75 and 125xc2x0 of arc, and a particularly preferred arc is 120xc2x0. Three scraping surfaces can then cover the full 360xc2x0 circumference.
The scraping surfaces can be connected by connecting rods or drillpipe made to the required length which are screwed together with a scraping body mounted at each screwed junction. The scraper bodies can be eccentric cylindrical bodies with internal splines and the connecting rods can have external splines on which the bodies are mounted against rotation. By assembling the scraper body in this way, it is possible to make up a scraper for various different applications, from component parts. For example, the scraper bodies can be exchanged for different diameter bodies to assemble a scraper for scraping a different diameter tube. A greater or lesser number of scraper bodies can be used depending on the tube diameter, the extent of cleaning likely to be necessary and other factors.
Each screwed junction can include a mounting surface for a scraper body, with part of the mounting surface having an external spline around its circumference and part being smooth around its whole circumference. Each scraper body has a central bore, one end of which can have internal splines and the other end of which can be smooth. This allows the angular orientation of the scraper body to be altered after the connecting rods threads have been engaged but before they have been fully tightened together. The scraper body can be mounted on the junction in any angular orientation and held in that orientation by engagement between the splines.
The splined part of the mounting surface at the junction between two connecting rods can be formed on one of the rods and the smooth part on the other rod.
According to a second aspect of the invention, there is provided a method of cleaning the inner surface of a tube of a predetermined internal diameter using a casing scraper which has an axis of rotation, a plurality of axially spaced, rigidly connected scraping surfaces with each surface having an angular extent of less than 180xc2x0 and being angularly offset from other surfaces, and shank portions rigidly connecting the scraping surfaces to one another, wherein prior to the insertion of the scraper in the tube, first distances, from the axis of rotation to each scraping surface, are greater than the radius of the internal diameter of the tube to be scraped, second distances from the axis of rotation to a surface diametrically opposite to each scraping surface, are less than the tube radius and the sum of the first distance and the second distance at each scraping surface is less than the internal tube diameter, the method including the step of flexing the shank portions as the scraper is inserted in the tube, so that when the scraper is inserted, the stored energy in the flexed shank portions presses the scraping surfaces against the tube internal wall.